CBSE Class 11 Chemistry Organic Chemistry Notes Set 05

These revision notes on Organic Chemistry: Some Basic Principles and Techniques for CBSE Class 11 Chemistry cover the classification of organic compounds, IUPAC nomenclature, fission of covalent bonds, carbocations, carbanions and free radicals, nucleophiles and electrophiles, inductive, resonance and electromeric effects, hyperconjugation and the main types of organic reactions, with the key structures drawn out for quick revision.

Quick Revision Notes

1. Classification

On the basis of functional groups, organic compounds are classified into following families or homologous series:

2. Nomenclature of Organic Compounds

Steps involved in IUPAC nomenclature are as follows:
Step 1 Locate the longest chain with as many possible secondary functional groups along with multiple bonds.
Step 2 Select the root word corresponding to the length of longest possible chain.
Step 3 Number the longest possible chain according to the rules to give an identification number to each C-atom of longest possible chain.
Step 4 Add suitable prefixes and suffixes along with numerals to indicate the number and position of each side chain substituent or functional group present in compound.
Hence, according to these rules the IUPAC name of a compound can be written as:

3. Fission of a Covalent Bond

(i) Heterolytic cleavage or heterolysis
X—Y → X⁺ + Y⁻ (heterolysis; when X is less electronegative than Y)
X—Y → X⁻ + Y⁺ (heterolysis; when Y is less electronegative than X)
Heterolytic cleavage results in formation of two types of species. These are:

(a) Carbocation or carbonium ion If the carbon has sextet of electrons and a positive charge, then the species is called carbocation (earlier called carbonium ion), e.g. CH₃⁺ (methyl carbonium ion).
It is trigonal planar with C-atom carrying positive charge is sp²-hybridised.
The order of stability of carbocations is
CH₃⁺ < CH₃CH₂⁺ < (CH₃)₂CH⁺ < (CH₃)₃C⁺
or methyl carbocation < 1° < 2° < 3°.

(b) Carbanion If the carbon of the obtained species is negatively charged and have octet of electrons in its valence shell, then the species is called carbanion. It is pyramidal in shape with atom carrying negative charge is sp³-hybridised.
The order of stability of carbanions is:
(C₆H₅)₃C⁻ > (C₆H₅)₂CH⁻ > C₆H₅CH₂⁻ > allyl carbanions > CH₃⁻ > (1° > 2° > 3° carbanions)

(ii) Homolytic cleavage or homolysis This cleavage involves movement of only one electron, which is represented by half headed (fish hook) curved arrow.

The neutral chemical species, thus formed are called the free radicals. It is a planar species in which C atom bearing odd electron is sp²-hybridised. Like carbocations, the order of stability of alkyl free radicals is

4. Nucleophiles and Electrophiles

(i) Nucleophiles or nucleophilic reagents An electron rich species having a lone pair of electrons or is negatively charged species which attacks on electron deficient areas is called a nucleophile (Nu:⁻).
Negatively charged species like H⁻, Cl⁻, Br⁻, I⁻, carbanions, OH⁻, ⁻OR, ⁻SR, CN⁻. Nucleophiles can also be seen in the form of neutral molecules with lone pair, e.g.

(ii) Electrophiles or electrophilic reagents An electrophile is defined as electron deficient species which attacks on electron rich areas, e.g. positively charged species like H⁺, Cl⁺, Br⁺, I⁺, NO₂⁺, SO₃ and carbocations.
The electrophiles can also be seen in the form of neutral molecules, e.g. carbenes (:CR₂), nitrenes (:NR), BF₃, etc.

5. Inductive Effect (I-effect)

In a polar covalent bond, the electrons are shifted to the more electronegative atom and hence, a polarity is developed in the bond. e.g. In a chain of carbon atoms having C—Cl bond, the C attached directly to the Cl-atom because of its less electronegativity acquires some positive charge (δ⁺).

This induction of polarity due to presence of polar bond in an organic molecule is called inductive effect.

Types of Inductive Effect
(i) Negative inductive effect Atoms or groups having greater electron affinity than hydrogen are said to have −I-effect (electron attracting). Some of the −I-effect producing groups in decreasing order of inductive effect are

(−I power in decreasing order with respect to H.)
(ii) Positive inductive effect Atoms or groups having lower electron affinity than hydrogen are said to have +I effect (electron releasing).
The order of +I-effect of some groups is as follows: 3° > 2° > 1° or CH₃.

6. Resonance Structure

These structures are hypothetical and individually don’t represent any real molecule. Benzene is a resonance hybrid of following two canonical forms

7. Resonance Effect (R-effect)

The polarity developed in a molecule due to interaction of two π-bonds or of a π-bond with the lone pair of electrons present on a fix atom is called resonance effect.
(i) Positive resonance effect (+R-effect) When the electrons move away from the atom or substituent group attached to the conjugated system, the effect is called positive resonance effect. In this effect, electron density increases at certain places, e.g. groups like halogen, —OH, —OR, —OCOR, —NH₂, —NHR, —NR₂ show +R-effect.
(ii) Negative resonance effect (−R-effect) When the electrons are transferred towards the atom or substituent group attached to the conjugated system, the effect is called negative resonance effect, e.g. >C=O, —CN and —NO₂ show −R-effect.

8. Electromeric Effect (E-effect)

The polarity developed due to complete transfer of a shared pair of π-electrons to any of the atoms linked directly to the multiple bond in the presence of an attacking reagent is called the electromeric effect.
(i) Positive electromeric effect (+E-effect) If the electrons of the π-bond are transferred to that atom of the double bond to which the reagent gets finally attached, then that effect is known as +E-effect, e.g.

(ii) Negative electromeric effect (−E-effect) If the electrons of the double (π) bond are transferred to an atom of the double bond other than the one to which the reagent gets finally attached, then that effect is known as −E-effect.

9. Hyperconjugation

The stabilising interaction that involves delocalisation of σ-electrons of C—H bond of an alkyl group linked directly to an atom of unsaturated system or to an atom having unshared p-orbital, is called hyperconjugation.
CH₃CH₂⁺ has empty p-orbital, with which one of the C—H bonds of the methyl group align in its plane and thus, the electrons of the C—H bonds can be delocalised into the empty p-orbital as shown below.

Orbital diagram showing hyperconjugation in ethyl cation

10. Types of Organic Reactions and Mechanism

The organic reactions can be categorised into following five categories:
(i) Substitution reactions

These are further classified as follows:
(a) Free-radical substitution The reaction brought by free radicals are called free radical substitutions, e.g. halogenation of alkanes.

(b) Electrophilic substitution reactions

(c) Nucleophilic substitution reactions These are usually written as S_N (S-substitution, N-nucleophile) and are of two types, i.e. unimolecular (shown as S_N1) and bimolecular (shown as S_N2).
These reactions are generally seen in alkyl halides and alcohols.
e.g. RX + KOH(aq) → ROH + KX

(ii) Elimination reactions An elimination reaction is one that involves the loss of two atoms or groups of atoms from the same or adjacent atoms of a substance leading to the formation of a multiple (double or triple) bond.
Elimination reaction is given by those compounds which have a nucleophile as a leaving group, i.e.

Elimination reactions are of two types:
(a) α-elimination reactions
(b) β-elimination reactions
e.g.

(iii) Addition reactions Reactions which involve combination between two reacting molecules to give a single molecule of the product are called addition reactions.
Such reactions are of typical compounds containing multiple bonds.

(iv) Addition-elimination reactions These reactions involve addition of two molecules with elimination of smaller molecules like H₂O, e.g. esterification.

(v) Rearrangement reactions These reaction involve the migration of an atom or a group from one atom to another within the same molecule.